Metal finishing method



y 9, 1953 M. B. VORDAHL ETAL 2,639,244

METAL FINISHING METHOD Filed July 15, 1950 2 Sheets-Sheet 1 NITRE BLACK WASH, LO D RACK SKKSodiumA (itrme WATER IOZSodium/lydrazid 940E minimum E "W l5-60/776flutes 5 mz'flutes Z. 9. i2. CLEANING BATH DRY HOT OIL zoayzlzefi HEATED CHAMBER 20F: 30 minutes WITH 0 FAN 500 F'. 20 minutes 3. 8. 13. RI N SE RlNSE, Cold Flowing HOT WATER DRAIN Water with 2oato 2|2F. Agitation 3 to Sma'fiqtes 4, 7. 14-. RINSE, WASH, I HOT WATER T B UNLOAD RACK 208to 2l2FZ l8OE/mmmum I 5minutcs Remove when turbulence subsides 5'. 6. DRY, CYANIDE BATH HEATED CHAMBER GOZSoaZzZumCyanz'de INVENTORS' WITH FAN 4OZPotssiumCyanide M/LTOA/ B. I/O/iDAhL 500'F. 946E minimum Y ROBERT h. GHA CE 20 minutes lot'oq-sma'nutes 210A //.CCL$7'0A/ 2 Sheets-Sheet 2 3 5. $3 mwmw @ZMGE V M5250 ZEWwEbm May 19, 1953 M. B. VORDAHL ETAL METAL FINISHING METHOD Filed July 15, 1950 K L 5 W o m o M H MM 2 8 mm 2 mm 8 E 3 8m m MA M w m RR 5/. R V L 0 9% am 0 W NT 5 owm M H L U 0mm 0 0mm mm 0mm M W 1 82 82 w 82 82 wZSu 5:8 02.52: I 32 N AU 82 82 u a one. 89 m 00: 0o: 3 ON: Ow: L 3 am 35:5 8 Son l 3N wmvw mv 3. 3:\ mm? 52 3% 93m ogn i NEW Q a a a om: v Y ow:

o m 2 9 cu mw om mm 2 3 8 mm om mm on R 8 3 om mm 3. E: 53 2 $255.6 H=z o 225cm Patented May 19, 1953 UNITED STATES PATENT OFFICE 2,639,244 METAL FINISHING METHOD Milton'B'. Vordahl; Trumbull, count, Robert H. Grace, Athol; Mass and Paul H. Eccle'ston,

Ilion; Nu-Y assignors to Remington Ai'ms com pany; Inc Bridgeport, Conn, 21 corporationof' Delaware Application J 13' I5, 1950; Serial No; 174,1]62' IWCIaims;

This application is'a continuation-in-part of our prior applications of the same title, Serial Number 41,826, filed July 31, 194B} and-Serial Number"'1l3,3'i2 filed August 31, 1949', both now abandonedi Theadditions and'corre'ctlons made in this application are the result of furtherexterisive production and laboratory experience with "this method of finishing.

This invention relates to animproved finish for metallic'articles and to a method of applying such a'finish. The invention has been foundtobe particularly effective in black finishing articles formed of .ferrousmetal; Theinvention provides a useful finish for firearms and parts thereof, Wire, strip, chain and other articles.

The obj ects of this invention are the provision ofan attractive, durable, and corrosion resistant finish for metallic articles which may be conveniently and economicallyapplied on a production" basis.

An'additional object is the development of a process which will produce uniform attractive coatings upon iron" orsteel', in'cluding'the socalled' stainless steels andwhich maybe appliediwith equally good results, to nickel, cobalt; andmanganese, or' to alloys including 50% or more by Weight of'one or more of'themeta'ls in thefgroupconsisting of iron, cobalt, nickel, and manganese;

Hretofore; the firearms finish which most nearly; met the requirements of the industry'has been that applied by the time-honored method of browning. This" processm'aybe briefly outlined as one in which a' polished, thoroughly clean item subjected to v the following treatment:

1. Plug or rustproofinterior surfaces.

Coat exterior with a rustproducing acid.

3L Allow rusting to proceed in a high humidity atmosphere;

4. 13611 to convert the-red oxide to a black oxide.

5. Wirebrush or otherwise remove any nonadherent oxide.

6; Repeat each ofsteps 2,13, 4, and at least once ancl'for as many as 5 or B cycles with very high grade arms.

, 7. Dye the relatively porous oxide coatingrand oil.

I The uality of the finish is dependent upon the degree of initial polish and cleanliness of the workpiece. The depth of color and the durability of the finish are largely dependent upon the number of cycles of rusting, conversion and carding, which is the name applied by the arms industry to the step: of wire brushing to'remove non-adherentoxides. This process is time con- 2. sumingand' requires an excessive amount of hand work. Since it is a-rusting process', it is-almost wholly ineffective when attemptsare made to apply it to rustless or stainless steels;

There are a number of-fast processes for producing blueor black finishes on firearmsbut in one way or another such processes areinadequate for high quality work. For example, the phosphate coatings widely used on military firearms have a dead greyoi dull black" appearance. The so-called oxide black processes which involve boiling the parts in water solutions, including various caustic-s and/or'nitrates or nitrites ofthe alkali metals; produce reasonably attractive finishes which have but little" durability or' corrosion resistance;

This invention resides in the discovery that polished; thoroughlyclean'articles, such as firearms, wire; or continuous strip, may be'treated atrelatively' low'temperaturs in a'molten bath of one"or more alkali metal cyanides by aprocess which except forthe low temperature is analogous tothewell-known cyanide hardening processes followed, after suitablecleaning, by treatment in a relatively low" temperature molten salt bath composediprincipally of an alkali metal'nitrate and which may includealso the hydroxide of an alkali metal. This process produces an exceptionally attractive, durable; andcorro'sion resistantfinish which may"beapplied to all surfaces of the material. The process is particularly economical since it" does'not require the" handling of individual pieces butp'ermit's small parts to be loaded in suitable racks holding several hundred items and inovedby suitable conveyors through the successive steps; The nature of the process also permits continuous operation with materials in the form of wire; strip, or chain.

We have found thatfthe process is efi ective when applied toiron', nickel; cobalt and manganese and to alloys ill-Which 50% or more by weight is of one or more of the metals selected from the above group; In any alloy of thisgroup the presenceof small amounts of copper appears to improvethedepth and richness of the color which in all 'ca'sesis"substantiallyblack. It may benfoted'j however, that'pure'nickel and some of the high nickel alloysshowasli'ghtly brownish cast and that variations inproces'sin'g', particularly relative to the temperature" of the nitrate bath permit variations in density of the color ranging, for example, with" carbon steel, from a semi-transparent-blue black to a very dense black.

The processcannot be'appliedto-pure molyb de- The exact nature of the invention, a well as other objects and advantages thereof, will more clearly appear from consideration of the following specification, referring to the drawing, in which:

Fig. 1 is a flow sheet outlining the preferred batch process of handling relatively small parts such as gun components.

Fig. 2 is a graph on which melting point and cost have been plotted for various percentages of sodium and potassium cyanide.

The specific description following will, in one section, discuss the invention in its application to firearms and parts thereof as a batch process. In another section a continuous process will be briefiy described. It must, however, be recognized that the process and finish secured thereby are essentially the same regardless of the specific article or the use to which it may be put.

Batch process The nature of the batch process is such that large quantities of parts may be handled simultaneously. Thus, for most efiicient operation, it is contemplated that suitable fixtures or racks may be employed and transported from step to step by means of a monorail and hoist travelling thereon. Suitable racks hold, for example, 225 firearm barrels or 640 fiream receivers. The design of such racks is not particularly critical, but the followinggeneral points should be observed. For uniform coloring, the racks should support the work with the minimum of surface contact to enable the active agents to reach all surfaces. Each rack should include a shield to deflect back into the furnace any material expelled as a result of placing a load of hot work therein. For greatest economy in the operation of the process, the racks should present a' minimum of surface area'and no re-entrant pockets which might trap salt and add materially to the inevitable dragout from the baths. In repeated use of the racks there is a tendency for the cyanide to'creep into pin holes in welded areas which causes small but violent explosions when the rack is placed in the nitrate bath, thereby causing deterioration of welded joints. It is therefore preferable to design the racks to avoid any welds below the line of maximum'immersion in the salts.

Clean Unlike the browning process, our process does not mask surface irregularities by filling in depressions. Accordingly, the metal surface should be prepared initially by grinding, polishing, blasting or other means to secure the desired surface finish and to remove any scale or rolling skin.

After the parts have been loaded into the racks the first step is a cleaning or degreasing'operation. Satisfactory results have been obtained by the use of alkaline type cleaners such as water solutions of 3 to 6 ounces per gallon of sodium metasilicate with about of a suitable wetting agent such as alkyl lauryl jsulfonate or by water solutions of proprietary alkaline cleaners under- 4 stood to contain caustics, sodium pyro or metasilicate, sodium carbonate, a wetting agent, and possibly other ingredients.

Such cleaning solutions may be used in a tank deep enough to cover the load of Work by at least three inches and may be maintained at a temperature at or near boiling. A range of 208 to 212 F. has been satisfactory. Mechanical agitation of the solution is preferable to insure thorough cleaning and it is desirable to move the rack completely in and out of solution several times at intervals during the cleaning treatment. A period of from ten to thirty minutes, depending on the type and amount of dirt, oil, etc., to be removed, is generally sufiicient to insure thorough cleaning. Particular attention should be paid to relatively inaccessible surfaces such as the bore of a barrel and if difficulty is found in achieving thorough cleaning, it may be necessary to Wipe out the bore with a suitable cleaner as a preliminary to placing the work in the racks. Execessive time in alkaline cleaning bath beyond that necessary for thorough cleaning should, however, be avoided as in several instances poor work has been traced back to this cause.

The use of vapor degreasing solutions such as trichlorethylene has also been found to be satisfactory. When this type of cleaning is used, it is practical to eliminate the subsequent two rinses and the drying operation and go directly from Clean to Cyanide. It is essential that the work be clean before coloring is commenced if satisfactory results are to be obtained.

Rinse Upon removal from the cleaner, the rack load of parts is immersed in a cold Water rinse tank to remove any adhering cleaner and to serve as a check on cleanliness. It Will be generally sat-' isfactory to examine the work for water breaks at this point, and if none are found, it may be assumed that the preceding cleaning step has accomplished its purpose. Otherwise, the cleaning treatment should be repeated. To remove adherent cleaner, it is advisable to move the work up and down several times. There should be continual circulation from an inlet at the bot-. tom of the tank and overflow at the top, and it is helpful to materially increasethe flow for one or more minutes during immersion to assist in sweeping the surface of the tank free of floating cleaner so that as the work is removed it will not be recoated with cleaning compound.

Hot rinse The cold water rinse may be followed by a hot water rinse in a similar tank provided with steam heating coils. For example, in one installation such a tank is permitted to idle at a temperature of around F. and normally overfiows enough to keep the water clean. When the rack of work is placed therein the inflow of water is stopped and the temperature permitted to rise to or near the boilingpoint. A range of 208 to 212 F. has been found satisfactory and a time of immersion" of 5 minutesis adequate.

Since the work is now chemically clean, any but the stainless materials will be quite susceptible to rusting and should not be exposed to air until the 5 minute immersion period is over, whereupon the work should be removed and transferred to the drier as quickly as possible.

Dry

intoithe: molten, cyanide: bathqalthoughmsz pres viouslv noted, itmay; be; omitted: if vapor dc.- greasing: equipment: is: used The; drier is essentially agas orotherwise heated; cabinet; which may. be maintained atyatemperature, of. approximately. 300 F. and-providedlwitha blower to insurea,rapidcirculaticnf air. As noted-above. speed is desirable at this stage of the process and the work should he placed in the drier and dried. as quickly as. possible. The heat of? the work. asit comes .fromthe hot: rinseaids in dry- Cyamda After drying, the work, may, be removed; from the drier andmarefullv inspectediforlwater, rust, streaks,. dirt or other, undesirable material If nfoneof these arepresent. the work may be transe ferredto .the. not, of. the. cyanide. furnace. After slowly, lowering the work into the, molten cyanide it should be moved; up and down until. thereiis no, longer any] encrustation of frozen. cyanide thereon. This'allows uniform operation from one load to the next; despite changes in load size. The work may then be. allowed to. remain immersed for, 10',to 45 minutes at .atemperature of notless than 9,40 F. The time. is dependent upon a, number, of factors including, process economics, the quality of the workoutput; andflthe cyanate content. ofthie bath. In regard to process economics, itrnay bevnotedthat 3. to 5, minutes in cyanide is usuallyadequate to prepare even stainless. steel forireceivingthe, black color. If color alone, is the criteria, suchshort treatment is adequate and, it would be unnecessary to use the more costly longer treatments. As to quality, it may be noted that thesuperlativewear and CO1? rosion resistance achieved-by this process will not usually be attained Without-.atleast 10. to minutes in the cyanide bath. After, about 45 minutes ,in cyanide the, carbon steels and other less resistant materialsare apt to be etched some? what and longer times do not 7 appear. beneficial to either appearance, wear, or corrosionresistance, of the coating- For the same quality of work thetime in cyanideappearsto vary, in a roughly inverseratioto the. cyanate' contentof the bath. Thus. in the range of 151% to cyanatje abofut 1.5. minuteswill produce a product equivalent to that produced. in 5% cyanate after about, 8.0 minutes.

For loads of) work. involving, a considerable weight of material; it is desirable to, preheat .the furnace. above. 940F. byv about 2? F. for eachlO pounds of .work before, immersing the workgto provide .a thermal head. and, to thereby avoid reducing, the, temperature materially below 9.40? F, as a resultof heat transfer tothe work. Alternate. methods of achieving. the same end, are the use ofoversized furnaces or the provision of heating devices capable of providing a temporarily stepped-up heat inputcomparableto the heat transfer to the work.

A suitable furnace must be. capable of immersingthe largest load of work to, be processed and should provide someroom for expansion of the cyanide salts. A preferred composition for the bathis ,thelow meltingeutectic mixtureof sodiurnv cyanide and potassium ,cyanide in proportions respectivelyv of 60 and140 parts by weight. However, the alkali, metal cyanide, istheactive material and other proportions or, a ,single,sa1t mavbeusedwith good results if the variations in melting. point are suitably, observed, Fig. 2 isa. graph in which the proportions, of. sodium andpotassium cyanide are plotted against melt hug-point tandfzpriceaofa the mixed saits; ftwillibe noted that: the.-v cheaper: sodium. cyanide has a much higher meltingpointthan the eutectic mixture and therefore the cost of heating the bath and the likelihood of drawing the temper of heat-treated' parts is-inoreased. As noted; we pref er. the eutectic mixture but, under conditions where heating costs: are 1 lower and heat damage not a= factor, sodium cyanide may be preferable. For heat treatedparts such as most firearmcomponents, atemperature-pf lOSG F. is about the maximum which can be tolerated without draw ing the temper.to amundesii'able extent. It may he noted that use-of the bath results in increasing cyanatecontentand reduces the melting point below that indicatedon the curve-which is'based on-the-ireslr salts:

Tocompensatefor-drag-outand-toobtain uni form activity of the bath, we have-found that it isdesirable to add 60% sodium cyanide and 40% potassium cyanide salts to the bath on a" daily basis. It should also be noted that maximum wear" and corrosion resistance is not attained until? a fresh bath has been aged at. working temperaturefor approximately two weeks: This aging; appear to have the function" ofbuilding up thecyanate content (ONO) of the bath to;a'degreeof-"5'% or'higher. After this degree ofcyanate-contenthas been reached; efforts shouldybedirectedat. minimizing the oxidation; responsible for cyanate build-up to avoid etching of. work, racks andfurnace pot. It appears to be desirableto holdflthe cyanate content below, 20% or the workmaybeetched toa degree which cannotbe: tolerated. Atightly fittingcover for the: cyanide bath is of assistance in reducing cyanate build up' and is' necessary to prevent water and other, materials splashing into the cyanide bath and to cutdown on heat loss. Floating covers which prevent the-access ofair to the surfaceof thecyanide have also proven useful.

As the bath is used, certain materials therefrom form an encrustation on the sidewallsof the pot above the fluid salt level and also on thewfioating coversiif these are used. Chemical analysisshows thismaterial to be predominantly sodiumwcarbonate, with some cyanide, and cyanate of sodium andrthe iron content has been found tobe .ashigh as. 0.28%. Indicationsare that. if this material is ermitted to return, to the.moltennbathtrouble will be, experienced in subsequent coloring. Accordingly, each morning this encrustation' is collected with a suitable skimming and scraping implement, and discarded.

Another, control technique which appears to. be effectiveis. a Weekly; sludgingpf the cyanidebath also somewhat reduced temperature. Accord ingly, each week-(end .the temperature may be re ducedes far aspossible. without loss. of fluidity in the, bath, and-,beforestarting p again the sludge which has settled L to.v the bottom. of the bath. is:dipp ed.out with suitably shaped dippers. In-thistwav, 25 to,50 poundsof sludgemay be removed per week.

Tins low;temperature cyaniding produces. with each of gthe materials towwhich the process may be; appliedi aathin. case. hardened surface not usually: over about. .OOOlinch thickuand, inaddition, has some effect not. fully understood which increases by several times the'wearand corrosion resistance of the black finish subsequentlyv applied thereto. Stainless steel and certain other" materials such as Stellite' and Cyanide wash Upon completion of the time cycle in the cyamde bath, the work should be raised and lowered several times, allowed to drain briefly, and moved to the wash tank. When heated rifle barrels or other tubular members are lowered vertically into a water bath, an effect analogous to the action of the well-known percolator causes a jet of water to spurt therefrom. To guard against this, a cover is placed upon each rack before washing unless the rack design is such as to inherently provide for deflecting this water back to the wash tank. Unless the cyanide furnace is remote from the wash tank, suitable precautions should be taken to prevent splashing Waterthereinto.

It should be noted that it is quite difficult to remove adhering cyanide if it has once been allowed to cool and dry on the work. Accordingly, an immediate and thorough washing is desirable. Hot or boiling water has been generally satisfactory as a washing agent, but about 3% sodium hydroxide or a small percentage of one of the alkaline cleaners added to the wash water is beneficial. The main objective is to get the surface as near chemically clean as possible.

It has been found to be undesirable to wash in cold water as the thermal shock tends to cause warpage of barrels and other precision parts, and, of course, the removal of adhering salts is more difficult. A desirable minimum temperature for the wash water has been found to be 180 F.

The cyanide wash tank is an installation which may be cited as an example is normally maintained in an over-flowing condition at a minimum 'of 180 F. When the work has been fully immersed, the temperature is allowed to rise to boiling, largely by heat transfer from the work, to assist in removing adhering salts. After turbulence has ceased, the work may be removed for transfer to the succeeding hot rinse.

Hot rinse A further rinsing-is sometimes desirable after the cyanide wash, and this rinse should also be in hot water. For example, this step may be carried out in a tank maintained between about 208 and 212 and slowly overflowing. The rack of work may be placed therein for a period of from 3 to 5 minutes, being moved up and down several times at the beginning and end of the cycle.

Work transfer As noted previously, it is desirableto avoid the use of racks with welded joints beneath the salt line. If it has not been possible to do this, it may be found that repeated use will have caused deterioration of the welded joints resulting in small cracks or pits which may entrap cyanide salts and retain them through the washing step. If welded racks having such joints are used, it is preferable to transfer the work after rinsing to a rack which is used only in the subsequent steps. Otherwise, local explosions of some violence may result when the racks containing entrapped cyanide are placed in the nitre bath. to follow. If this transfer is made, clean gloves should be worn to prevent contamination of the work.

D11) Whether or not the work is transferred to another rack, it is thoroughly dried, and this opera-'- tion may be performed conveniently in a drying cabinet substantially the same as that used after cleaning. For fast drying the cabinet may, for example, be maintained at substantially 300 F. for substantially 20 minutes and provided with a blower to produce a current of heated air.

Nitre The nitre bath is a molten salt bath in which the active ingredient is a nitrate of an alkali metal with which a caustic alkali or other derivatives may be used. We prefer to use a nitre bath which consists initially of parts by weight of sodium nitrate and 10 parts of sodium hydroxide. It is this bath which imparts the black color to the work, and experience has shown that the shade of the color is to a considerable extent dependent on temperature. For example, 900 F. yields a reddish black product, 950 F. a blue black, and 1000" F. a dark black. The color is not satisfactory for firearm components if the temperature is allowed to drop during coloring below 940 F. It is therefore desirable topreheat the bath or in some other way provide a thermal head sufficient to offset heat transfer to the work. For example, with a furnace holding between 1600 and 2000 pounds of molten salt, it has been found sufficient to preheat the bath before receiving the work by about 2 R, for each 10 pounds of work to be placed therein. Thus, with a load of 150 barrels, weighing about 445 pounds, it is desirable that the nitre bath cited as an example be preheated to about 1040" F., or a raise of F. By so controlling the thermal head, it is possible, and desirable, to insure that the work, during most of the time it is immersed in the bath, will be maintained at a temperature within the range of about 940 F. to 1050 F.

As in the case of the cyanide bath, a maximum temperature of 1050 F. should be observed for heat treated parts to prevent drawing the temper to an undesirable extent.

All work must be thoroughly dry before being placed in this furnace, and upon removal from the drier a careful inspection must be made to verify that this is so. The work may then be slowly lowered into. the bath but the lowering should not be interrupted unless absolutely necessary until the parts have been completely covered, or a ring on the work may result. After complete immersion, the work may be moved in and out of the bath until the Work seems to have taken on its deepest color, which will be after the work has come fully up to temperature. The work may then be all-owed to remain in the bath for a total period of from not less than about 15 minutes and preferably 30 to 60 minutes, after which it may be raised and lowered several times. As the work is removed from the furnace pot, it may be inspected for color. If any areas of a hard red oxide are noted, the Work should be returned to the furnace for another 20 minutes.

Although we have above noted our preference for a bath of 90% sodium nitrate and 10% sodium hydroxide, it may be noted that the caustic alkali may be omitted with only slight detriment to the color. It appears that the effect of the caustic is to produce a slight etching of the surface and to give the impression of a deeper richer black. The straight nitrate tends to produce a semi-transparent blue-black. The most general-' 1y. attractive finish will be produced after the caustic content has been raised to about 5%;

11 through a heated chamber arranged to accommodate one or two unit lengths of wire at a time. Y

Ten minutes appears to be an adequate length of time for the cyanide treatment of wire and may be accomplished at the previously mentioned v rate of travel by arranging the guides to hold one unit length of wire submerged at a time. Obviously or minutes will give a deeper cyanide case and is desirable with such material as bicycle chain. Such longer period may be provided for by arranging the guides to respectively hold two or three unit lengths of wire submerged at a time. v

Wire has very little tendency to drag out cyanide salts and the wash following cyanide treatment need not be elaborate. Probably the minimum is to draw the wire througha pad in and through which water is being circulated and the maximum would be to draw the wire through a hot or boiling water bath in which one or two unit lengths could be submerged at a time. With such material as bicycle chain, however, several unit lengths should be submerged in substantially boiling water and a rinse should beperformed in a second hot bath in order to ensure that the cyanide is completely removed. As before, the work may be dried by passing through a heated chamber.

Ten minutes in the nitre bath is usually adequate for wire and twenty to thirty should suffice for the heavier more complicated articles such as chain. As before, these times can be achieved at the previously discussed rate of travel by arranging the guides to hold one, two, or three units of length submerged in the bath at one time. The nitre wash maybe carried out in the same manner as the previously described wash steps, preferably in hot water, and the work then passed through hot oil unless oil would exert an undesirable effect on the end product as in certain electrical applications of such treated wire.

With wire or other material of substantial strength, the entire process may be handled by a single winding and reeling unit at the discharge end which pulls the material from the delivery spool and through all of the process baths. With fine wire it is desirable to use several winding units at intervals between the baths to minimize the tensile strain on the wire.

This process has been particularly effective upon several variations of Nichrome used in fine wire form as resistance materials. In this application, in addition to color, the wire is materially improved in regard to tensile strength and its electrical properties and the dielectric effect of the coating are modified, in a way we do not understand but which seems to be of great interest to the producers of Nichrome resistance wire.

We do not intend to indicate that the continuous treatment of wire, strip, or other continuous articles for coloring or heat treatment is novel with us but simply wish to point out that our process, developed originally for batch application to small mechanical devices mounted in racks, can be successfully applied to the treatment of wire and like articles. Accordingly, we wish to note that we consider the appended claims to apply to a continuous as well as to a batch process.

Effectiveness of the process Bythe batch process it has been possible to deep blue-black color of 'superior appearance which is characterized by very high wear, scratch, and corrosion resistance. For example, comparative tests of identical arms parts subjected to the prior art treatments and to this process have been made on the basis of measuring the time necessary to produce equal degrees of corrosion on exposure to the conventional salt spray test. Several of these tests are tabulated below. The material is identical in each case and the treatment of the sample is indicated in the left hand column. The time indicated is that necessary in each case to produce an equivalent degree of corrosion on all samples in that column.

Another comparative test which perhaps more accurately represents service conditions was performed with identical receivers for Model 512 Remington rifles. One set of samples was processed as disclosed herein and subsequently completely degreased to remove any of the oil which might have tended to mask the improvement in corrosion resistance due to the Cyanide-Black process. The other set of samples was processed by the conventional Browning technique and also degreased.

Both sets of samples were exposed in identical locations in the rusting room of the conventional Browning process which is accurately maintained at a constant F. temperature and 70 percent relative humidity. After three weeks, samples of the browned receivers were removed and found to be badly rusted. Sample Cyanide- Black receivers were removed after 2 /2 years exposure and showed no spots of rust or corrosion. At the present time Cyanide-Black receivers of this lot are still in the rusting room and, after more than three years exposure, still show no rust spots. A recent sample of browned test and except for surface treatment identical with the Cyanide-Black receivers still in the rusting room, was removed after 3-weeks and 3-days exposure and again the sample was badly rusted.

Further recent tests have also demonstrated the outstanding wear and corrosion resistance of the Cyanide-Black process. These tests were performed with a standard Taber abrasion and scratch tester. This machine provides a turntable upon which samples may be mounted and is arranged to be driven by suitable power. Over this turntable there is mounted an arm carrying a pair of rubber bonded abrasive wheels mounted so that the axis of the wheels is at an angle to the radius of the turntable. As the turntable rotates it is dragged under the wheel to the angular position of the wheel axes, a com ponent of the dragging force rotates the abrasive wheels at a relative slow rate. A revolution counter is provided to record the number of cycles to which each sample is subjected. The Wheels are standardized and are resurfaced after each 500 cycles of rotation of the turntable. The loading on the arm may be selected to determine the severity of the test and the test is concluded when the surface finish has been abraded suifi ciently to expose the underlying metal. In our and, due

test we used identical samples of 18-8 stainless steel and SAE l'701carbon steel. The carbon steel specimen and one stainless specimen were treated by the process there disclosed :and .the other stainless specimen was treated t-by :the best available competitive method of :blacking stain less steel.

In the comparative test, all "test :plates were thoroughly cleaned to remove aallitra'ces of oil and .dirt. Standard Taber -210 f.(..alibrase wheels were used for :abrading and a 500 gram load wassapplied tothewheels. After 100011370185 the competitive process sample of stainlesszsteel was clearly worn through, at 3000 cycles there was a clear break through withthe Cyanide- Black.carbonsteel-specimen, and at 5000 cycles the Cyanide-Black stainless sample showed only a -questionable break through.

In the scratch test with this same equipment the arm is changed to utilize an arm holding a carbide cutter somewhat like a lathe threading tool and the specimen is moved thereunder by small increments of turntable movement. After each increment the loading on the arm is increasedby 50 grams. The first break-through readily apparent byvisual'inspection of the sample. The specimens were prepared as for the abrasion test and at a 450 gram "loading the cutter penetratedthe finishof the competitive process stainless specimen. The scratch test machine will not take more than 1000 grams load on the cutter and at that load neither the stainless nor carbon steel Cyanide-Black specimens showed any visible evidence of scratching.

Since in the most economical application of this process to firearms barrels the bore of the barrel is left open during processing, this same wear and corrosion resistant surface will be applied thereto. As a result, the bore is rendered highly resistant to mechanical abrasion and to corrosion from the chemical agents released by the burning of certain types of propellent powder and primers. As an example, a 12-gauge shotgun having its bore surface finished in this way has been fired with 5,000 rounds of ammunition of all available types, with no evidence that the coating has been affected in any Way. Caliber .22 rifles have also been fired many thousands of rounds without evidence of mechanical wear or chemical corrosion of any surfaces.

The processes as presently practiced have been outlined in considerable detail, Obviously variations may be made therein without departing from the spirit of the invention and it should be clearly understood that it is intended to include within the scope of the appended claims all equivalent materials, proportions, and procedures. While extended experience with the process may indicate that not all of the control procedures outlined are necessary, their use appears to be advantageous for full scale production.

What is claimed is:

1. The process of securing a stable, uniform, substantially black coating upon an article having a surface of iron, nickel, cobalt, manganese, or of an alloy containing 50% or more by weight of one or more of said metals, said process comprising the steps of submerging the article in a first treating bath having as its essential ingredient a molten cyanide of at least one of the alkali metals at a temperature not less than about 940 F. for a period of time sufficient to develop a shallow hardened skin; and subsequently submerging the article in a second treating bath having as its essential ingredients 2.

molten nitrate oral; least oneofithe alkali metals admixed with vfrom 5% "to 15% of alkali metal hydroxide, said second -bath being maintained at a temperature not lessthan about 9e0 Furor a vperiod :of :time suflicient to develop th black color, sald first treating bath having "been aged at; substantially working temperatures and therebyppartially oxidized to'increase the cyanate content and thereafter controlled to maintain the cyana-te content in a range between about *five percent "and about twenty percent.

.2."Ihe :process as described in claim 1, said article :beingmaintainecl inzsaid first bath for a periodof :time not less than about "ten minutes.

3,..The :process :as :described in claim 2, .said

article being maintainedin'said second bath for a period of time not .less than about fifteen minutes. 4. The :proc'ess .as described in claim 3, said first and second baths'bein'g maintained for substantially the entire period of immersion .o'fsaid article in said baths in atemperaturerange between about 940 F. and about 1050"13.

5.'Th'e process as described in claim 4, said article being immersed "after removal from said first hathandagain after removal fromsaid secand bath in heat absorbing, adherent salt dissolving, aqueous media maintained at a temperature not substantially less than about 180 F.

The process 5&5 described :-in claim .5, .said first bath consisting initially of the product of the low melting eutectic mixture of sodium and potassium cyanides.

7. The process as described in claim 6, said second bath consisting initially of the molten product of the admixture of substantially percent sodium nitrate and substantially 10 percent sodium hydroxide.

8. The process of securing a stable, uniform, substantially black coating upon an article having a surface of iron, nickel, cobalt, manganese, or of an alloy containing 50% or more by weight of one or more of said metals, said process comprising the steps of chemically cleaning all surfaces of said article; submerging said cleaned article in a first treating bath having as essential ingredients a mixture of molten alkali metal cyanides maintained at a temperature between about 940 F. and about 1050 F. for a period of time between 10 and. 45 minutes; freeing said article of adherent cyanide by immersing in water maintained at a temperature not less than about 180 F.; subsequently submerging said article in a, second treating bath of a molten mixture including not less than about 65 percent molten alkali metal nitrate and not more than about 15 percent caustic alkali maintained at a temperature between. about 940 F, and about 1050 F. for a period of time between about 15 and 60 minutes; freeing said article of adherent alkali metal salts by immersing in water maintained at a temperature not less than about 180 F.; and subsequently submerging said article in a heated, protective, oily material.

9. The process of securing a stable, uniform, substantially black coating upon a firearm component having a surface of iron, nickel, cobalt, manganese, or of an alloy containing 50% or more by weight of one or more of said metals, said process comprising the steps of chemically cleaning all surfaces of said component; submerging said cleaned component in a first treating bath having as essential-ingredients a mix ture of molten alkali metal cyanides maintained at a temperature between about 940 F. and about 150 F. for a period of time between and 45 minutes; freeing said component of adherent cyanide by immersing in water maintained at a temperature not less than about 180 F.; sub sequently submerging said component in a secand treating bath of a molten mixture including not less than about 65 percent molten alkali metal nitrate and not more than about percent caustic alkali maintained at a temperature between about 940 F. and about 1050 F. for a period of time between about 15 and 60 minutes; freeing said component of adherent alkali metal salts by immersing in water maintained at a temperature not less than about 180 F.; and subsequently submerging said component in a heated, protective, oily material.

10. The process of securing a stable, uniform, substantially black coating upon the bore and exterior surface of a firearm barrel consisting of iron, nickel, cobalt, manganese, or of an alloy containing 50 or more by weight of one or more of said metals, said process comprising the steps of chemically cleaning all surfaces of said barrel; s'ubmerging said cleaned article in a first treating bath having as essential ingredients a mixture of molten alkali metal cyanides maintained at a temperature between about 940 F. and about 150 F. for a period of time between 15 and 45 minutes; freeing said barrel of adherent cyanide by immersing in water maintained at a temperature not less than about 180 F.; subsequently submerging said article in a second treating bath of a molten mixture including not less than about percent molten alkali metal nitrate and not more than about 15 percent caustic alkali maintained at a temperature between about 940 F. and about 1050 F. for a period of time between about 15 and 60 minutes; freeing said barrel of adherent alkali metal salts by immersing in water maintained at a temperature not less than about 180 R; and subsequently submerging said barrel in a heated, protective, oily material.

MILTON B. VORDAHL.

ROBERT E. GRACE.

PAUL H. ECCLESTON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name o Date 1,247,086 Crowe Nov. 20, 1917 1,528,245 Crapo Mar. 3, 1925 1,853,562 Herr Apr. 12, 1932 1,988,217 Sayles Jan. 15, 1935 2,370,959 Holden Mar. 6, 1945 2,431,986 Clingan Dec. 2, 1947 2,476,345 Zavarella July 19, 1949 2,513,302 Field July 4, 1950 

1. THE PROCESS OF SECURING A STABLE, UNIFORM, SUBSTANTIALLY BLACK COATING UPON AN ARTICLE HAVING A SURFACE OF IRON, NICKEL, COBALT, MANGANESE, OR OF AN ALLOY CONTAINING 50% OR MORE BY WEIGHT OF ONE OR MORE OF SAID METALS, SAID PROCESS COMPRISING THE STEPS OF SUBMERGING THE ARTICLE IN A FIRST TREATING BATH HAVING AS ITS ESSENTIAL INGREDIENT A MOLTEN CYANIDE OF AT LEAST ONE OF THE ALKALI METALS AT A TEMPERATURE NOT LESS THAN ABOUT 940* F. FOR A PERIOD OF TIME SUFFICIENT TO DEVELOP A SHALLOW HARDENED SKIN; AND SUBSEQUENTLY SUBMERGING THE ARTICLE IN A SECOND TREATING BATH HAVING AS ITS ESSENTIAL INGREDIENTS A MOLTEN NITRATE OF AT LEAST ONE OF THE ALKALI METALS ADMIXED WITH FROM 5% TO 15% OF ALKALI METAL HYDROXIDE, SAID SECOND BATH BEING MAINTAINED AT A TEMPERATURE NOT LESS THAN ABOUT 940* F. FOR A PERIOD OF TIME SUFFICIENT TO DEVELOP THE BLACK COLOR, SAID FIRST TREATING BATH HAVING BEEN AGED AT SUBSTANTIALLY WORKING TEMPERATURES AND THEREBY PARTIALLY OXIDIZED TO INCREASE THE CYANATE CONTENT AND THEREAFTER CONTROLLED TO MAINTAIN THE CYANATE CONTENT IN A RANGE BETWEEN ABOUT FIVE PERCENT AND ABOUT TWENTY PERCENT. 